204 is a wonderful rabbit hole! You can use it to send messages to the server without refreshing the page without JS. It was a pretty common trick in the early 2000s
Send an actual error page rather than a 204. It's not perfect, but in cases where the server returning an error is very rare, it's fine. Since there is no state anyway, the user hitting back is fine.
Most JS apps in my experiences fail to handle server errors with any sort of user feedback at all.
